Radar level gauges are used for making non-contact measurements of the level of products such as process fluids, granular compounds and other materials. These devices utilize an antenna or a probe to transmit electromagnetic waves toward the material being monitored and to receive electromagnetic echoes which are reflected at the surface of the material being monitored. The relationship between transmitted and received waves is then used to determined the location of the reflection, i.e. the surface.
In a pulsed radar level gauge, constant power electromagnetic pulses are transmitted into the tank with a repetition frequency typically in the range 100 kHz to a few MHz. The pulses can be DC pulses or modulated by a microwave frequency carrier wave. The pulse can be guided by a wave guiding structure into the tank, or be allowed to propagate freely. On the receiver side, a reflection from the interior of the tank is received, and a low frequency analogue tank signal is formed and then digitized to form a digital time domain reflectometry (TDR) signal. The location of a surface echo is determined by identifying peaks in this TDR-signal using amplitude detection.
However, amplitude detection as the only method for identifying the location of an echo in the tank signal suffers from limitations. The reason is that the peak is in fact the envelope of a waveform, making a small shift of the waveform very difficult to detect. In order to obtain a satisfactory result, the envelope needs to have a sharp peak, leading to strict requirements for pulse modulation of the signal when relying on amplitude detection. In addition to this, the resolution of A/D converter needs to be relatively high, as limitations in system causing saturated signals may further reduce measurement performance. The situation is illustrated in FIG. 1, showing a waveform representing a reflection of a modulated pulse being slightly shifted a time δ between two locations, and the resulting, almost undetectable, difference in envelope maximum.